Experimental investigation of chamber‐dependent radical sources

Abstract

While environmental chamber data have been widely used to generate and validate computer models of the chemistry occurring in polluted atmospheres, the effects of the chambers on the gas-phase chemistry being studied have been poorly characterized. In order to investigate such chamber effects, a series of NO,-air irradiations, with trace levels of organics present to monitor OH radical concentrations, have been carried out in four different environmental chambers (ranging in volume from -100 to 40,000 L) at varyiing temperatures, humidities, pressures, and reaction conditions. In addition, a number of control experiments have been carried out to validate the technique for measuring OH radical levels in these irradiations. The data show that unknown sources of OH radicals are present in all of the chambers studied. The data are consistent with the presence of two distinct radical sources: (1) the photolysis of initially present HONO, whose importance increases with increasing N O D O concentration ratios, but which is a minor contributor to the overall radical flux after 30-60 min of irradiation, and (2) a constant (for these NO,-air irradiations) radical source which dominates beyond approximately the first 60 min of irradiation. The radical input rates, after the first -30-60 min of irradiation, are independent of the h10 concentration, increase with increasing temperature, humidity, and NO2 concentration, are proportional to light intensity, and are dependent on the chamber employed. Although the exact nature of this radical source is still undetermined, results of experiments reported here allow a number of possible mechanisms to be ruled out, and these are discussed.

An investigation of the dark formation of nitrous acid in environmental chambers

“…In particular, in a series of environmental chamber experiments designed to measure OH radical levels in NOx·air irradiations, we observed an initial, rapidly-decaying radical source [6,7]. This was almost certainly due to the photolysis of initially present HONO, since direct in situ spectroscopic measurements of HOND using the DOAS technique agreed well with values calculated to fit the observed OH radical concentration-time profiles [7].…”

“…However, the amount of HONO formed prior to irradiation in our 5800-L evacuable chamber appeared to increase with the N0 2 concentration, the relative humidity and the temperature, but be independent of (or indeed decrease with) the initial NO concentration [7]. These observations showed that the heterogeneous reaction (3) wall NO + N0 2 + H20~2 HONO could not be responsible for this observed formation of HONO under these conditions, but suggested that a reaction of N0 2 with water was occurring [7].…”

“…However, if HONO is also formed by some nonphotolytic, nonradical reaction, then its subsequent rapid photolysis would represent a net source of OH radicals both in the atmosphere during the period around sunrise [4] and during the initial stages of environmental chamber irradiations [6,7].…”

“…In environmental chamber systems, HONO photolysis is believed to contribute, at least in part, to the excess rate of formation of radicals characteristic of those systems [6,7], which complicates the use of environmental chamber data for testing computer kinetic models of photochemical smog formation [11]. In particular, in a series of environmental chamber experiments designed to measure OH radical levels in NOx·air irradiations, we observed an initial, rapidly-decaying radical source [6,7].…”

Catalytic Role for Water in the Atmospheric Production of ClNO

“…9,[12][13][14] This hydrolysis reaction merits computational studies for several reasons. First, nitrous acid Figure 1.…”